[Development] More on QDateTime / QTimeZone

[John Layt]

On Thursday 12 Sep 2013 13:48:24 Thiago Macieira wrote:
> On quinta-feira, 12 de setembro de 2013 21:06:03, John Layt wrote:
> > * Choose either local msecs or epoch msecs, if epoch then that change
> > can't
> > be  done for 5.2
> 
> Choose the easiest for you to implement. We're running out of time, so I'd
> rather stick to the implementation you have and your best recommendation.

I've spent the evening writing and running some benchmark tests (amazingly we 
didn't have any before).  Each benchmark creates a date a day for 10 years and 
calls the tested method, so 3650 consecutive dates.  The numbers here are from 
1000 iterations on my i7.  As expected they're a mixed return with some things 
faster and some things slower (not all tests shown):

Action                      Base    Msecs     %
--------------------------  -----   -----  ----
create():                   0.294   0.392    33
isValid():                  0.339   0.492    45
date():                     0.313   0.632   102
time():                     0.311   0.639   105
timeSpec():                 0.311   0.403    30
offsetFromUtc():            7.466   9.009    21
toMSecsSinceEpoch():        6.690   8.836    32
toMSecsSinceEpoch1950():    7.522   3.642   -52
toMSecsSinceEpoch2050():    8.618  10.790    25
setDate():                  0.418   0.832    99
setTime():                  0.385   0.820   113
setTimeSpec():              0.340   0.508    49
setOffsetFromUtc():         0.347   0.438    26
setMSecsSinceEpoch():       6.020   2.564   -57
addDays():                  0.701   1.129    61
addMonths():                1.836   2.270    24
addYears():                 1.826   2.255    23
addMSecs():                12.820  10.370   -19
toTimeSpec():               6.900   9.247    34
toOffsetFromUtc():          7.251   9.292    28
daysTo():                   0.365   0.953   161
msecsTo():                 13.510  17.780    32
equivalent():               0.367   0.439    20
equivalentUtc():            6.730   9.018    34
lessThan():                 0.355   0.444    25
lessThanUtc():              6.714   8.906    33
currentDateTime():          1.118   0.314   -72
fromMSecsSinceEpoch():      6.094   2.451   -60


* Things that start with an msecs value (i.e. use localtime) are 60-80% faster
* Things that start with a date and time (i.e. use mktime) are 30% slower.
* Focusing on optimising toMSecsSinceEpoch() will cause most other methods to 
speed up, if I remove a tzset call it runs faster than the baseline.
* The getting and setting of the date and time are obviously slower as they 
now involve conversions.
* Rather strangely timeSpec() is 30% slower despite now only returning the 
member directly instead of having to do a switch based on the member, a few 
other calls are equally confusing.

Cheers!

John.